1. Field of the Invention
The present invention relates to a drive force distribution control system for use with a motor vehicle, and more particularly to a drive force distribution control system for controlling a ratio between drive forces to be distributed to a pair of laterally spaced drive road wheels of a motor vehicle.
2. Description of the Relevant Art
Motor vehicles include power transmission mechanisms for transmitting power from a drive source to a pair of laterally spaced drive road wheels to produce drive forces. Such a power transmission mechanism includes a differential for driving the drive road wheels on respective outer and inner turning circles at respective speeds dependent on the radii of these outer and inner turning circles while the motor vehicle is making a turn. Conventional differentials are arranged to distribute equal amounts of power from the drive source to the drive road wheels. Therefore, if one of the road wheels sticks in a muddy pit or the like in a road, it is idly rotated, and the amount of power transmitted to the other road wheel is reduced so that effective drive forces cannot be produced. To avoid this difficulty, there have recently been proposed various differentials which can limit the differential function to maintain predetermined minimum drive forces available for propelling the motor vehicle. Differentials with the ability to limit the differential function, often called limited-slip differentials, include a torque-proportional differential for limiting the differential function dependent on a torque acting between a pair of pinions, and a preloaded differential utilizing the resiliency of a spring.
The above conventional differentials mechanically limit the differential function. When the motor vehicle makes a turn, the drive force produced by the drive road wheel on the inner turning circle is increased owing to the difference between the rotational speeds of the drive road wheels on the inner and outer turning circles. The increased drive force then generates a moment in a direction to prevent the turning of the motor vehicle, and hence the turning performance of the motor vehicle is lowered.